<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var a;
    var b = null;
    console.log(a);  //undefined
    console.log(a==b);  //true
    console.log(a===b);  //false
    console.log(b+10);   //10
    console.log(a+10);   //NaN
</script>
</body>
</html>